(1) Field of the Invention
The present invention relates to a power semiconductor device suitable for an electric power application circuit which is represented by an inverter for use in an industrial power electronic appliance or a home electrical appliance.
(2) Description of the Related Art
In recent years, there have been strong demands for higher power, higher efficiency, and downsizing of electric power application circuits. An inverter, which is one of the representative electric power application circuits, is made up of a plurality of basic blocks in each of which a diode is connected in parallel with a collector and an emitter of a power transistor. This diode is used to let a reverse current escape when the transistor is turned OFF, and especially has a performance requirement of reducing a time of switching from ON to OFF. When the switching is not sufficiently fast, a problem of shorting between a high side and a low side of the inverter circuit arises. To prevent this, a Fast Recovery Diode (FRD) with excellent high-speed performance is typically used.
The FRD is required to have both high breakdown voltage characteristics in reverse bias and low on-resistance characteristics in forward bias. In general, there is a tradeoff relation between a breakdown voltage and an on-resistance of a diode, and a limit of the tradeoff is determined by physical properties of a semiconductor used for the semiconductor device. Diodes made of Si have conventionally been employed as such semiconductor devices, but in recent years the development of power semiconductor devices using wide-bandgap semiconductors such as SiC and GaN is ongoing to achieve higher performance.
In particular, an AlGaN/GaN heterostructure of nitride semiconductors has a high dielectric breakdown electric field and a high sheet carrier concentration when compared with conventionally-used silicon, and therefore can realize both high breakdown voltage characteristics and low on-resistance characteristics. For this reason, diodes with this structure have captured attention as high-power FRDs. Studies of diodes having AlGaN/GaN heterojunctions, which are intended for use as high-power semiconductor devices, have been reported from a number of organizations in Japan and abroad.
A high two-dimensional electron gas concentration of the order of 1013 (cm−2) can be obtained by using an AlGaN/GaN heterostructure. To reduce the on-resistance even further, a diode having a plurality of heterojunctions has also been proposed (for example, see Japanese Unexamined Patent Application Publication No. 2006-108676).
The following describes a conventional diode made of nitride semiconductors, with reference to FIG. 1. In FIG. 1, a buffer layer 30, a GaN layer 39, an AlGaN layer 41, a GaN layer 38, and an AlGaN layer 40 are disposed on a substrate 28 in this order, and a Schottky electrode 20 and an ohmic electrode 22 are formed on the AlGaN layer 40 which is an outermost layer of an epitaxial structure. First two-dimensional electron gas is induced at an interface between the AlGaN layer 40 and the GaN layer 38, and second two-dimensional electron gas is induced at an interface between the AlGaN layer 41 and the GaN layer 39. A plurality of channels created by such two-dimensional electron gas serve to reduce the on-resistance.
However, since the Schottky electrode 20 has a junction with the AlGaN layer 40, when the diode is forward biased, a resistance between the Schottky electrode 20 and the first two-dimensional electron gas is relatively low, but an access resistance to the second two-dimensional electron gas increases due to a high heterobarrier. Consequently, the total on-resistance cannot be reduced as much as expected, despite the plurality of channels.
Moreover, since the Schottky electrode 20 has a junction with the AlGaN layer 40, a parallel-plate capacitor is created between the Schottky electrode 20 and the two-dimensional electron gas. This increases a capacitance of the diode. As a result, the diode cannot be switched between ON and OFF at high speed.
In view of the above problems, the present invention has an object of providing a semiconductor device that has a sufficiently low on-resistance and excellent low-capacitance and high-speed characteristics as compared with conventional GaN-based diodes.